This invention relates to cellular mobile radio-telephony systems and, more particularly, to a hand-off operation wherein a communication channel, normally used for voice communication, is also used for monitoring the strength of signals transmitted from a mobile radio-telephony station of a mobile subscriber in order to select the next cell into which the mobile subscriber is to be handed off.
A system of interest in understanding celullar mobile radio is disclosed in a U.S. patent application entitled "Cellular Mobile Radio Service Telephone System" of S. O. Goldman et al having Ser. No. 457,155, filed Jan. 11, 1983 now U.S. Pat. No. 4,562,572, and assigned to the assignee hereof, which application is incorporated by reference herein in its entirety.
Cellular radio-telephony is used at numerous sites in this country and overseas. Such. a telephone system permits communication between two mobile telephone stations as well as between a mobile telephone station and a fixed station.
Such systems are formed of clusters of cells wherein each cell is assigned to a predetermined set of voice frequency channels. The channels are spaced apart in the frequency spectrum so as to permit simultaneous transmission of many telephone conversations by many stations without interference between communications on the various channels. In order to insure that there is no interference between the assigned frequency channels of one cell and the assigned frequency channels of a contiguous cell, the individual channels in the contiguous cell are located at different portions of the frequency spectrum than the individual channels of the one cell. The same frequency channels are repeated at more remote cells, and the power of the signal transmission in any one channel is limited in amplitude so as to become attenuated to a sufficiently low, non-interfereing level, at the frequency channels of the remote cell.
Cellular mobile radio-telephone systems are described in the literature. One such system referred to as an "Advanced Mobile Phone Service" is described in The Bell System Technical Journal, January 1979, Vol. 58, No. 1, pp 1-269. Multiplexing of individual subscriber channels for communication via RF (radio frequency) link is accomplished, preferably, by means of statistical multiplexers. Such multiplexers are described in an article entitled "Controlling Data Communications: Statistical Multiplexer Moves In" by H. J. Hindin in Electronics, July 28, 1981, pp. 141-148, and in "A Buyers Guide to Today's Volatile Statistical Multiplexers" by J. H. Sharen-Guivel and A. A. Calson in Data Communications, March 1982, pp. 97-126. A switching configuration for a mobile system is disclosed in "A Distributed Switching Approach to Cellular Coverage" by R. E. Pickett in Telecommunications Magazine, February 1983. A network control system for use in cellular mobile radio-telephony may include the commercially available ITT System 1210 hardware and software.
In the construction of a cellular system, a group of the foregoing cells is clustered about a system switching network which dynamically allocates the available frequency channel in any one cell among the various mobile radio-telephones with which communication is desired at this instant. Such switching networks provide for the coupling of a telephone conversation of one frequency channel in a first cell with a second frequency channel in a second cell or, alternatively with a long distance trunk circuit which connects the first cell with a distance trunk circuit which connects the first cell with a desired cell in another cluster or with a fixed station. In addition, well-known control circuitry is provided for the transmission of command signals to the mobile stations for directing their respective transmissions on the allocated frequency channels.
As a mobile station moves from one cell to the next cell, a hand-off procedure is followed wherein the central switching network commands the mobile station to switch frequency from the channel which was used in the first cell to the frequency of a new channel to be used in the second cell. A characteristic in hand-off decision-making circuitry presently in use is the measurement of the amplitude of signal transmission with the mobile station. The communication system may include directional antennas at each cell site, the antennas designating specific azimuthal sectors showing generally the position of a mobile station within a cell. Thus, the signal strength or quality can serve as an indication that the mobile transmitter is centrally located within a cell, or is located near the boundary bf the cell. Thereby, by monitorting the amplitude or quality of such signal transmissions the decision-making circuitry of the hand-off apparatus is able to signal the system switching network at the appropriate time when a hand-off is to be made from one frequency channel to another frequency channel.
Under present regulations of the FCC (Federal Communcations Commission), cellular mobile radio-telephone systems comprising a set of cells are allocated 333 channels of which 21 channels are used for access and control purposes, such as the paging of a mobile station, while the remaining 312 channels are used for the transmission of voice and data. For example, the geographic area covered by the system may be divided into a cluster of fifteen cells having approximately twenty voice channels apiece, or a cluster of twenty cells having approximately fifteen voice channels apiece. The communication channels in each of the cells operate as part of a common network under a network control system in which the twenty-one access channels are available for tasks including the servicing of call attempt from and paging of mobile stations. Each voice channel would be shared on a statistical basis among numerous subscribers, such sharing being possible because not all subscribers wish to communicate at the same time. Thus, each channel may handle 20-30 subscribers just as a trunk line is shared among many subscribers.
A problem exists in presently configured cellular mobile radio systems in service due to the fact that, in cellular mobile systems presently in use at least one transceiver in each cell is reserved for purposes such as the locating of a mobile station, and therefore is not available for use in the communication of voice and data between subscribers. The mode of hand-off operation employed in present cellular mobile system necessitates the reservation of the foregoing locating transceiver in each cell of the system.